1. Technical Field
The present invention relates to a reflective liquid crystal display (LCD) having a light diffuser.
2. Background of the Invention
Reflective LCDs, in which incident light from the viewer""s direction is reflected for display, have been proposed. FIG. 1 is a cross sectional view of such a reflective LCD.
The reflective LCD shown in FIG. 1 comprises thin film transistors (hereinafter referred to as xe2x80x9cTFTsxe2x80x9d), which function as switching elements, on an insulating substrate 10 composed of a quartz glass, non-alkali glass or the like.
More specifically, on the insulating substrate (TFT substrate) 10, gate electrodes 11 comprising a refractory metal such as chromium (Cr) or molybdenum (Mo), a gate insulating film 12, and an active layer 13 comprising a polycrystalline silicon film are sequentially formed in that order.
In the active layer 13, channels 13c are formed at positions directly above the respective gate electrodes 11, and a source 13s and a drain 13d are formed at outer sides of the channels 13c by ion doping using stopper insulating films 14 above the respective channels 13c as a mask.
An interlayer insulating film 15, formed by accumulating an SiO2 film, an SiN film and an SiO2 in that order, is disposed over the entire surface covering the gate insulating film 12, the active layer 13 and the stopper insulating films 14. Then, a metal such as aluminum is used to fill a contact hole located corresponding to the drain 13d, thereby forming a drain electrode 16. A planarization insulating film 17 composed of an organic resin or the like is further disposed so as to cover the entire films located below for surface planarization. Another contact hole is formed in the planarization insulating film 17 at a position corresponding to the source 13s, and a reflective display electrode 19 composed of Al is formed on the planarization insulating film 17. The reflective display electrode 19 contacts with the source 13s via this contact hole and also serves as a source electrode 18. Then, an alignment film 20 composed of an organic resin such as polyimide for performing alignment of liquid crystal 21 is disposed on the reflective display electrode 19.
On an opposing electrode substrate 30 composed of an insulating substrate which faces the TFT substrate 10, at the side of the TFT substrate 10, a color filter 31 comprising red (R), green (G), and blue (B) filters and black matrixes 32 having light shielding function, a protecting film 33 composed of a resin, an opposing electrode 34, and an alignment film 35 are sequentially formed in that order. Further, on the other side of the opposing electrode substrate 30, a retardation film 44 and a polarization film 45 are disposed. The opposing electrode substrate 30 and the TFT substrate 10 are adhered to each other at the periphery using a sealing adhesive (not shown) to sandwich twisted nematic (TN) liquid crystal 21 in the gap formed therebetween.
The path light travels when the above-described reflective LCD is viewed will next be described.
Referring to FIG. 1, as indicated by the dotted line with an arrow, natural light 100 enters the device from the polarization film 45 provided at the side of an viewer 101, transmits sequentially through the retardation film 44, the opposing electrode substrate 30, the color filter 31, the protecting film 33, the opposing electrode 34, the alignment film 35, the TN liquid crystal 21, and the alignment film 20 on the TFT substrate 10, and is then reflected by the reflective display electrode 19. The reflected light then passes through these layers in the reverse order and direction, and is emitted out of the device from the polarization film 45 disposed on the opposing electrode substrate 30 to reach the eyes of the viewer 101.
Referring now to FIGS. 2A and 2B, the luminance measurements of reflective light in a reflective LCD will be described.
FIG. 2A depicts a method for measuring luminance of a surface of a reflective LCD and FIG. 2B shows the measurement results.
As shown in FIG. 2A, a reflective LCD panel comprising a TFT substrate 10 and an opposing electrode substrate 30 is disposed with the display surface located above. Light entering the display panel is made to do so at a predetermined angle of inclination xcex8in with respect to the normal direction of the display surface. This incident light 105 is reflected by a reflective display electrode. A light intensity detector 106 measures the reflected light emitted from the display panel at predetermined emission angles. More specifically, the light intensity detector 106 is moved to a position having an angle of xcex8out with respect to the normal line (indicated by dotted line) of the LCD panel of FIG. 2A to detect the reflected light at the angle xcex8out for intensity measurements.
The measurement results are shown in FIG. 2B by dotted lines. In FIG. 2B, the horizontal axis indicates a detection angle of reflected light and the vertical axis indicates the intensity of reflected light at respective detection angles.
However, as indicated by the dotted lines in FIG. 2B, a reflective LCD of the type described above is disadvantageous in that high intensity light is only reflected at certain detection angles, such that over a wide range of the display panel, bright display cannot be achieved.
In order to overcome this disadvantage, providing a light diffuser between the protecting film 33 and the opposing electrode 34 on the opposing electrode substrate 30 has been considered.
The relationship between the emission angle and the intensity of reflected light when the light diffuser is provided is also shown in FIG. 2B by a solid line. As shown, compared with the results shown by the dotted line, light with intensity can be obtained over a wider variety of angles, in other words, over a wider range, and bright display can be achieved when the light diffuser is provided.
However, at an angular range of xcex81 in FIG. 2B, the intensity of reflected light becomes low, and brightness of the display changes abruptly. This causes non-uniformity of brightness when the viewer changes the viewing angle from the normal direction to the horizontal direction. Thus, these proposed reflective LCDs still can not overcome the disadvantage that brightness of display is not uniform and depends on the viewing angle.
The present invention was made in view of the foregoing disadvantages of the related art, and aims to provide a reflective liquid crystal display (LCD) capable of achieving uniformly bright display with increased luminance for each display pixel.
In accordance with one aspect of the present invention, there is provided a reflective liquid crystal display device, comprising liquid crystal provided in a gap between first and second substrates disposed facing each other, and electrodes for driving the liquid crystal each provided on the first and second substrates at the side opposing the liquid crystal, wherein, of said electrodes, an electrode formed on one of said first and second substrates is a reflective display electrode composed of a conductive reflective material, said reflective display electrode includes, at least on a surface opposing the liquid crystal, a concave portion depressed toward said first substrate in each pixel region, and said concave portion includes a base portion and a slope portion inclined toward said base portion.
In accordance with another aspect of the present invention, in the above reflective liquid crystal display device, said reflective display electrode is formed on an insulating film having a portion concave toward said first substrate.
In accordance with still another aspect of the present invention, in the above reflective liquid crystal display device, said reflective display electrode is formed on an insulating film having a portion concave toward said first substrate, over a switching element formed for each pixel.
In accordance with further aspect of the present invention, in the above reflective liquid crystal display device, said first or second substrate is provided with a light diffuser.
In accordance with yet another aspect of the present invention, in the above reflective liquid crystal display device, the extent of diffusion of said light diffuser has the haze value of between 19% and 70%.
In accordance with further aspect of the present invention, in the above reflective liquid crystal display device, the angle of elevation of said slope portion with respect to said base portion is greater than 0xc2x0 and 8xc2x0 or less.
As described above, in each pixel region, the reflective display electrode includes the slope portion in addition to the flat base portion, to thereby provide surfaces oriented at different angles with respect to the incident light. It is therefore possible to reflect the incident light in an effective range without loss to thereby achieve bright display with high contrast. For example, by setting the angle of elevation of the slope portion as described above, the incident light can be reflected within a very effective range of emission angle.
Further, by providing the light diffuser, the light which is effectively reflected by the above-mentioned reflective display electrode can be uniformly emitted toward the viewer. In addition, when a light diffuser with optimal characteristics according to the size of the display, for example, is employed, a bright image free from display non-uniformity (luminance non-uniformity) can be obtained in various display devices of different screen sizes.
As described above, according to the present invention, it is possible to provide a reflective LCD capable of achieving increased luminance in each display pixel and of providing bright display over a wide range of viewing angles.